Structural and functional connectivity phenotype in the neocortex of Fmr1 ^−/y mice supports a prominent hypothesis of autism.

Key points: Creatine (Cr) plays an important role in muscle energy homeostasis as a substrate in the creatine kinase phosphoryl exchange reaction, but the consequences of creatine depletion are incompletely understood. We assessed the morphological, metabolic and functional consequences of systemic creatine depletion on skeletal muscle in a mouse model with deficiency of an essential enzyme in the biosynthesis of creatine (AGAT^−/− mice). We show that Cr depletion leads to several metabolic abnormalities in muscle, including reduced ATP, increased inorganic phosphate levels and reduced activities of proton‐pumping respiratory chain enzymes and an elevated glycolytic contribution in ischaemic circumstances. The Cr‐depleted muscle suffers from reduced grip strength, severe atrophy and abnormal mitochondrial structures, increased overall mitochondrial content and an increased number of lipid droplets. Oral Cr administration led to rapid accumulation in skeletal muscle (faster than in brain) and reversed all the muscle abnormalities, revealing that the condition of the AGAT^−/− mice can be switched between Cr deficient and normal simply by dietary manipulation. Abstract  Creatine (Cr) plays an important role in muscle energy homeostasis by its participation in the ATP–phosphocreatine phosphoryl exchange reaction mediated by creatine kinase. Given that the consequences of Cr depletion are incompletely understood, we assessed the morphological, metabolic and functional consequences of systemic depletion on skeletal muscle in a mouse model with deficiency of l‐arginine:glycine amidinotransferase (AGAT^−/−), which catalyses the first step of Cr biosynthesis. In vivo magnetic resonance spectroscopy showed a near‐complete absence of Cr and phosphocreatine in resting hindlimb muscle of AGAT^−/− mice. Compared with wild‐type, the inorganic phosphate/β‐ATP ratio was increased fourfold, while ATP levels were reduced by nearly half. Activities of proton‐pumping respiratory chain enzymes were reduced, whereas F₁F₀‐ATPase activity and overall mitochondrial content were increased. The Cr‐deficient AGAT^−/− mice had a reduced grip strength and suffered from severe muscle atrophy. Electron microscopy revealed increased amounts of intramyocellular lipid droplets and crystal formation within mitochondria of AGAT^−/− muscle fibres. Ischaemia resulted in exacerbation of the decrease of pH and increased glycolytic ATP synthesis. Oral Cr administration led to rapid accumulation in skeletal muscle (faster than in brain) and reversed all the muscle abnormalities, revealing that the condition of the AGAT^−/− mice can be switched between Cr deficient and normal simply by dietary manipulation. Systemic creatine depletion results in mitochondrial dysfunction and intracellular energy deficiency, as well as structural and physiological abnormalities. The consequences of AGAT deficiency are more pronounced than those of muscle‐specific creatine kinase deficiency, which suggests a multifaceted involvement of creatine in muscle energy homeostasis in addition to its role in the phosphocreatine–creatine kinase system.